The physiology of platelet secretion has many features in common with the secretory physiology of endocrine and neuronal cells; and a number of the biogenic amines sythesized, stored, and secreted by these different cell types are similar. Platelet membrane glycoproteins (GP) appear to be major factors determining cell- cell recognition, and secretion. We have identified a new epitope of GPIb that acts as a receptor for von Willebrand's factor which controls platelet endothelial interaction but does not interfere with platelet secretion or reaction with drug antibodies that also interact with GPIb. The role of platelet production in the pathogenesis of idiopathic thrombocytopenic purpura (ITP) has been controversial. We studied platelet turnover in ITP with 111In-labeled autologous platelets and found approximately normal production rates in all 15 cases. We found that a platelet- specific antibody, anti-P1E1, that we identified in a patient with Bernard-Soulier Syndrome, reacted with the glycoprotein Ib complex and glycocalacin and inhibited ristocetin-induced platelet agglutination. We have found that the anion transport blockers, SITS, DIDS, and suramin inhibit exocytosis of platelets by interfering with the action of agonist on platelet receptors rather than by inhibiting anion flux.